The etching of silicon wafers by "plasma" or "dry etch" technique is well known in the art. A silicon wafer comprises a layer of a first material on top of a layer of a second different material, with certain portions of the first material exposed to the plasma through a mask. The wafer is subjected to the plasma etching process. The aim of the plasma etching process is to etch away the first material until the second material is exposed without etching any of the second material. Thus, a critical factor is to determine when the plasma etching process should terminate.
One of the prior art methods for monitoring the etching of a silicon wafer is to monitor the amplitude intensity of the optical emission from the plasma discharge. Many processes have particular spectra lines or regions which are driven by the presence of some chemical constituents in the plasma such as a reactant species or a plasma etch product produced as an etching byproduct. By monitoring the intensity at the wave length of these species in the plasma, the concentration of those species in the plasma, which is directly related to the status of the etching process, can be determined.
Recognizing, however, that the monitoring of a single spectra line does not necessarily result in the exact determination of when the second material has been completely etched away exposing the first material, another prior art method teaches the monitoring of a narrow band centered about a predetermined spectra line which is indicative of the gas phase concentration of the plasma etch reactant or product of the plasma, and a wide band centered about a predetermined spectra line indicative of the optical background emission signal. Other art describes adding a reference filter to the main signal filter which compensates for changes in the plasma due to things other than film clearing. The drawback of this method is that it assumes a non-zero main filter output signal; this is not always the case. Because the change in the RF match at the end point can artifically dampen or strengthen a single end point filter output, the drawback of the prior art technique is that they have not been highly reproducible.